Update the openstack_sd_config section

openstack_sd_config requires a 'role' parameter which wasn't documented.

Remove wrong statement about alertmanager URL configuration

[minor] docs: recording_rules: fix missing key

Fixes https://github.com/prometheus/docs/issues/681

'Prometheus lifecycle' should be a subsection of 'Miscellaneous'

Change wrong URL for remote.proto

Resolves an issue where rendered markdown was incorrect.

Small changes to migration guide

* Initial draft of migration.md

* Edits.

* Review feedback.

* Review feedback.

* Staleness link to video; add docker root example; remote config file section.

* s/NB/NOTE/, remove external labels link.

* More typos.

* Add more details link for removed PromQL features.

* s/you/your/

* Expand on prom1.8/2.0 side by side setup.

* More feedback.

* update links.

* --query.lookback-delta flag.

This endpoint has moved to /api/v2 (with somewhat different properties)
in Prometheus 2.0 and should now be part of a separate admin API page.

* Add 2.0 storage docs

* Review fixups

* More review fixups

It's unclear why this is in the example configuration file. Probably best to keep that super simple, c.f. https://github.com/prometheus/docs/pull/895#discussion_r148924390

* promu: Use same go invocation as prometheus' for building

If you override $(GO) var in prometheus Makefile in order to use
a custom go binary you might encounter errors while building promu
as promu's Makefile $(GO) var is also defined to look for go in the
$PATH and you might end up using different go binaries for building
prometheus and promu.

This patch makes prometheus Makefile pass its $(GO) var to promu's makefile
as environmental variable.

Signed-off-by: Sylvain Rabot <s.rabot@lectra.com>

* Makefile: Allow $(GO) to be set with environmental variable

Signed-off-by: Sylvain Rabot <s.rabot@lectra.com>

* Makefile: Fix hardcoded call to gofmt

Signed-off-by: Sylvain Rabot <s.rabot@lectra.com>

* typo in prometheus.yml field causes prometheus to throw an error

Fixes #3414

Cut 2.0.0-rc.3

This showed up all of a sudden, even if checking out older commits.

Guard against tags being nil in EC2 discovery

Fixes #3001

Update vendoring for tsdb and clean it up in general